1. Field of the Invention
The present invention relates to a method of polarizing a piezoelectric body, and more particularly, to a method of polarizing a piezoelectric body having a structure in which a plurality of piezoelectric layers and internal electrodes are alternately laminated, wherein the piezoelectric layers on both sides of each internal electrode are polarized in opposite directions.
2. Description of the Related Art
Conventionally, a piezoelectric resonator has been provided in which design flexibility of characteristics is substantial, a spurious response is small, and the difference xcex94f between the resonance and anti-resonance frequencies can be increased (Japanese Unexamined Patent Application Publication No. 10-4330). The piezoelectric resonator includes a plurality of piezoelectric layers and internal electrodes which are alternately laminated, and the piezoelectric layers on both sides of each internal electrode are polarized in opposite directions. For piezoelectric resonators having such a structure, the polarization degrees of the piezoelectric layers have a substantial effect on the characteristics of the resonator. Therefore, the amounts of scatter in polarization degrees within each device and between devices must be minimized as much as possible.
The polarization treatment of this type of monolithic piezoelectric bodies is carried out as shown in FIG. 1. A piezoelectric body 1 is a block-shaped piezoelectric ceramic. Here, the piezoelectric body 1 composed of four piezoelectric layers 1a to 1d is shown for explanation, and may include at least five layers. Internal electrodes 2a to 2c are provided between the piezoelectric layers 1a to 1d, respectively. The internal electrodes 2a to 2c are led out alternately to the outer surfaces of the piezoelectric body 1 and connected to external electrodes 3 and 4, respectively. By applying a direct current electric field between the external electrodes 3 and 4, the piezoelectric layers 1b and 1c on both sides of the internal electrode 2b are polarized in opposite directions, respectively, as indicated by the arrows P to obtain a predetermined polarization degree.
However, according to the method shown in FIG. 1, the electric field is concentrated at the end portions of the internal electrodes 2a to 2c, which causes the polarization degree distribution to be non-uniform. FIG. 2 shows an example of the polarization degree distribution, in which the slanted lines represent the polarization degrees. As seen in FIGS. 1 and 2, when the electric fields are applied to the piezoelectric body 1 in the thickness direction, the polarization degree is substantially increased in the four corners of the piezoelectric body 1. Thus, a uniform polarization degree distribution cannot be obtained. As a result, if the piezoelectric body having such a non-uniform polarization degree distribution is cut in substantially rectangular elements for use as completed devices, the peripheral portions of the piezoelectric body cannot be used. That is, the use range (yield of the piezoelectric body) is substantially limited.
To overcome the problems described above, preferred embodiments of the present invention provide a method of polarizing a piezoelectric body in which the polarization degree distribution of a monolithic piezoelectric body produced by the method is substantially uniform, and the yield is, therefore, greatly improved.
According to a first preferred embodiment of the present invention, a method of polarizing a piezoelectric body having a plurality of piezoelectric layers and internal electrodes which are alternately laminated, includes the steps of applying an electric field to the piezoelectric body to polarize the body uniformly in the thickness direction thereof, and applying electric fields in the opposite directions to the piezoelectric layers on both sides of each internal electrode, respectively, whereby only the piezoelectric layer on one side of the internal electrode is polarization-inverted.
According to a preferred embodiment of the present invention, a method of polarizing a piezoelectric body having a plurality of piezoelectric layers and internal electrodes are alternately laminated includes the steps of applying a first electric field to the piezoelectric body to polarize the body uniformly in the thickness direction thereof, applying a second electric field in the reverse direction with respect to the direction of the first electric field, whereby the piezoelectric body is polarization-inverted uniformly in the thickness direction, and applying electric fields in the opposite directions to the piezoelectric layers on both sides of each internal electrode, respectively, whereby only the piezoelectric layer on one side of the internal electrode is polarization- inverted.
Further, according to a third preferred embodiment of the present invention, a method of polarizing a piezoelectric body having a plurality of piezoelectric layers and internal electrodes which are alternately laminated, includes the steps of applying electric fields in the opposite directions to the piezoelectric layers on both sides of each internal electrode, respectively, whereby the piezoelectric layers on both sides of the internal electrode are polarized in the opposite directions, and applying electric fields in the reverse directions with respect to the directions of the above electric fields to the piezoelectric layers on both sides of the internal electrode, respectively, whereby the piezoelectric layers on both sides of the internal electrode are polarization-inverted.
According to the first preferred embodiment of the present invention, after an electric field is applied to the piezoelectric electric body to polarize the body uniformly in the thickness direction, electric fields are applied in the opposite directions to the piezoelectric layers on both sides of each internal electrode, respectively, whereby the piezoelectric layer only on one side of the internal electrode is polarization-inverted. By polarization-inverting the piezoelectric body, the phenomenon in which the polarization degree is increased in the four corners of the piezoelectric body is substantially suppressed, and the non-uniformity of the initial polarization degree distribution is substantially reduced. Therefore, when the substantially rectangular elements produced by cutting the piezoelectric body are used, the available range is significantly widened and the yield is greatly enhanced.
FIG. 3 illustrates the change of a polarization degree distribution, caused by initial polarization and polarization-inversion.
By the initial polarization, the polarization degrees increase in the end portions of the piezoelectric body with a large difference xcex94P1 between the polarization degrees in the end portions and the approximate center portion. Subsequently, the polarization-inversion is carried out, so that the polarization axial direction is inverted, and simultaneously, the difference xcex94P2 between the end portions and the approximate center portion is reduced. The electric field intensity at the polarization-inversion is substantially equal to that at the initial polarization, and the polarization time is shorter than that of the initial polarization. The maximum polarization degrees Pmax at the initial polarization and the polarization-inversion are equal to each other.
FIG. 4 shows the variation of a polarization degree when positive and negative electric fields are applied to a piezoelectric body.
First, when a positive electric field is applied at point I, the polarization degree is increased to point II. When the application of the electric field is stopped, the polarization degree is stabilized at point III. The polarization degree at the point III is a residual polarization degree. Subsequently, when an electric field is applied in the negative direction, the polarization degree is decreased to substantially zero (point IV). After this, the polarization axis is inverted, and the polarization degree is lowered to point V. When the application of the electric field is stopped, the polarization degree is restored to point VI where the polarization degree is stabilized.
In FIG. 4, the polarization degree Pr obtained after the positive electric field is applied and the polarization Pr obtained after the negative electric field is applied so that the piezoelectric body is polarization-inverted, are opposite in polarization axial direction, and are substantially equal to each other (points III and VI), if the electric field intensities are substantially the same except for the polarities. In addition, the amount of scatter in polarization degree distribution is greatly reduced as shown in FIG. 3.
In the first preferred embodiment of the present invention, the piezoelectric body is polarized uniformly in the thickness direction, and thereafter, the piezoelectric layer on one side of each internal electrode is polarization-inverted. In the second preferred embodiment of the present invention, the piezoelectric body is polarized uniformly in the thickness direction, and is polarization-inverted at the same time. Thereafter, the piezoelectric layer on one side of the internal electrode is polarization-inverted again, whereby the non-uniformity of the polarization degree distribution is substantially reduced. Further, the second polarization-inversion to be carried out is easily performed.
The step of polarizing (or polarization-inverting) the piezoelectric body uniformly in the thickness direction is carried out on the piezoelectric body in the block state according to the first and second preferred embodiments of the present invention. This is desirable to achieve greatly increased productivity. The succeeding polarization-inversion of the piezoelectric layer only on one side of each internal electrode is carried out on the piezoelectric body in the block state or on the substantially rectangular elements produced by cutting the piezoelectric body. Especially, in the case where the substantially rectangular elements produced by cutting the piezoelectric body are polarization-inverted, the intensity of electric field and the application time is set corresponding to the polarization degree distributions of the respective substantially rectangular elements. Accordingly, adjustment of the polarization degree can be performed with increased precision.
According to the third preferred embodiment of the present invention, electric fields are applied in opposite directions to the piezoelectric layers on both sides of each internal electrode, whereby the piezoelectric layers on both sides of the internal electrode are polarized in opposite directions. Further, electric fields are applied in the reverse directions with respect to the directions of the above electric fields to the piezoelectric layers on both sides of the internal electrode, whereby the piezoelectric layers on both sides of the internal electrode are polarization-inverted. That is, the polarization axial directions of the piezoelectric layers on both sides of the internal electrode are inverted with respect to the first polarization axial direction of the piezoelectric layers. For this reason, any of the piezoelectric layers is polarization-inverted, so that the amount of scatter of the polarization degree is effectively reduced.
Preferably, the polarization process is carried out on the piezoelectric body in the block state, and an electric field in the forward or backward direction is applied to each substantially rectangular element after the piezoelectric body is cut into the substantially rectangular elements. Accordingly, the polarization degree is individually adjusted to be increased or decreased. By this, the amounts of scatter in polarization degrees within each substantially rectangular elements and between the substantially rectangular elements are even more reduced.
Other features, characteristics, elements and advantages of the present invention will become apparent from the following description of preferred embodiments thereof with reference to the attached drawings.